Hepatocellular carcinoma (HCC) is the fifth most common cause of cancer worldwide and the third most common cause of cancer related mortality (Jemal et al., 2011). Surgical resection and liver transplantation have long been the only two treatment options that offer the possibility of a cure for patients with HCC (El-Serag and Rudolph, 2007; El-Serag, 2011; Llovet and Bruix, 2008). Other locoregional therapies, however, including percutaneous ethanol injection and thermal ablation, now appear to be rivaling surgical resection in terms of efficacy (Lencioni, 2010; Liapi and Geschwind, 2010, Ann. Surg. Oncol.; Liapi and Geschwind, 2010, J. Hepatobiliary Pancreat. Sci.).
Unfortunately, only 10-15% of the patients with HCC are eligible for such treatments because of the advanced stage of the disease at the time of diagnosis, or the presence of underlying liver disease (Hong et al., 2006). One major obstacle to the effectiveness of any systemic treatment for HCC is the presence of co-morbid disease ranging from fibrosis to end stage liver cirrhosis.
Liver fibrosis is the scarring process that represents the liver's response to injury and it is a result of chronic inflammatory injury to the liver parenchyma. The primary causes of liver fibrosis are chronic hepatitis virus infection and alcohol and non-alcoholic steatohepatitis. Secondary causes of liver fibrosis include autoimmune conditions and genetic disorders (Friedman, 2013; Mallat and Lotersztajn, 2013). Liver fibrosis ultimately leads to cirrhosis, liver failure, and hepatocellular carcinoma, which results in serious morbidity from the clinical symptoms of cirrhosis (i.e., ascites, variceal bleeding, renal failure and hepatic encephalopathy) and an exceptionally high mortality for patients with HCC that is associated with end-stage liver cirrhosis.
The pathogenesis of liver fibrosis is a process involving the progressive replacement of the hepatic parenchyma with collagen rich extracellular matrix (ECM). In the chronically injured liver, repeated and overlapping phases of inflammation and wound-healing overwhelm the normal regenerative process and cause a net deposition of collagen. Chronic liver injury over an extended period of time leads to the end-stage disease characterized by a profound destruction of the liver lobule with thick bands of fibrotic tissue that bridge hepatic vessels and which surround nodules of regenerating hepatocytes. In gross terms, cirrhosis can be divided into two types: a micro-nodular meshwork of fibrotic bands separating small regenerative nodules and macro-nodular broad bands of fibrotic septae separating larger nodules of varying sizes. This observation suggests a dynamic disease process with several cellular events underlying the cirrhotic and eventual carcinogenic transformation active at the same time.
Recent data suggest that hepatic stellate cells (HSCs) are primarily responsible for fibrosis (Sanchez-Valle, 2012). In the normal liver, HSCs compose from about 5% to about 10% of cells and are located in the subendothelial space between hepatocytes and sinusoidal endothelial cells. During chronic liver disease, HSCs are activated to undergo phenotypic changes from a quiescent (referred to as a normal HSC) to a myofibroblastic phenotype (referred to as an activated HSC). Activation of HSCs is the central event in hepatic fibrogenesis, and it consists of early (“initiation”) and late (“perpetuation”) phases.
Parenchymal injury promotes activation of Kupffer cells (resident liver macrophages), endothelial cells and platelets, and an influx of leucocytes, resulting in the generation of lipid peroxides, reactive oxygen species, and a number of cytokines, such as TGF-β, interleukin-1, TGF-α, PDGF, and EGF (Ogawa et al., Liu et al., 2011). These factors promote induction of specific sets of transcription factors in HSCs within hours, resulting in the expression of proteins involved in fibrogenesis. Synthesis of TGF-α and TGF-β promotes activation of neighboring quiescent HSCs, whereas the release of HGF stimulates regeneration of adjacent hepatocytes. Accumulation of fibrogenic cells results from a high mitogenic and an enhanced capacity to escape from apoptosis. Mitogenicity is stimulated by growth factors, such as PDGF, which has great promitogenic effects, vasoconstrictors, such as thrombin, the matrix metalloproteinase MMP2, and adhesion molecules.
Further, interaction with matrix components, such as collagen I and fibronectin, also plays a crucial role in survival of activated HSCs. The profibrogenic potential of activated HSCs (hepatic myofibroblasts) is due to their capacity to synthesize fibrotic matrix proteins and components that inhibit fibrosis degradation. HSCs express a wide range of matrix metalloproteinases (MMPs), as well as MMP activators that cleave pro-MMP into their active form. In addition, they also produce specific tissue inhibitors of the metalloproteinase family (TIMPs).
Production of MMPs and TIMPs is tightly regulated according to the activation state of HSCs, and it reflects extracellular matrix remodeling during chronic liver injury. At early stages, HSCs express MMP1, MMP2, MMP3, and MMP9 and their activators, but do not produce TIMPs; this allows degradation of normal matrix in the subendothelial space and its substitution by fibrillar collagens. In contrast, fully activated HSCs shut down expression of MMPs and turn on expression of TIMPs, resulting in a dramatic reduction of collagenolytic activity and allowing for the production of fibrotic matrix.
Current therapeutic strategies for reversal of fibrosis and cirrhosis are based on inhibition or attenuation of HSC activation and target a single pathway or phase of the fibrotic progression (Liu et al., 2013; Schuppan and Kim, 2013). Hepatic fibrosis and carcinogenesis, however, is a complex multistep process that results in a large number of heterogeneous molecular changes that can be exploited as potential targets for therapy. An ideal anti-fibrotic drug should be liver specific to avoid adverse effects on extra-hepatic matrix proteins and should selectively attenuate excessive collagen deposition without affecting normal extracellular matrix synthesis.